Allow to make other (non .adp) kinds of resources "theme aware" and use this to provide a theme mapping to the /resources/ URL pattern, then use this new capability to allow the override of css in the chat implementations

Revert massive replacement of empty list creation sentences. The use of '[list]' instead of '{}' adds semantics that could be used for performance improvements in the future, such as using a different internal representation. There is already work in this direction, avoiding the generation of the string representation during comparison of empty strings (huge thanks to Stefan Sobernig for the pointer: https://core.tcl.tk/tcl/info/44527c632ed609c2).